1. Field of the Invention
The invention generally relates to the field of fabricating semiconductor devices. More specifically, it relates to the deposition of barrier films on porous dielectric material.
2. Prior Art
Modern integrated circuits generally contain several layers of interconnect structures fabricated above a substrate. The substrate may have active devices and/or conductors that are connected by the interconnect structure.
Current interconnect structures, typically comprising trenches and vias, are usually fabricated in, or on, an interlayer dielectric (ILD). It is generally accepted that, the dielectric material in each ILD should have a low dielectric constant (k) to obtain low capacitance between conductors. Decreasing this capacitance between conductors, by using a low dielectric constant (k), results in several advantages. For instance, it provides reduced RC delay, reduced power dissipation, and reduced cross-talk between the metal lines. For some cases, interconnect structures use dielectric materials such as silicon dioxide (SiO2) or silicon oxyfluoride (SiOF). Articles discussing low k dielectrics are: “From tribological coatings to low-k dielectrics for ULSI interconnects,” by A. Grill, Thin Solid Films 398–399 (2001) pages 527–532; “Integration Feasibility of Porous SiLK Semiconductor Dielectric,” by J. J. Waterloos, et al., IEEE Conference Proceedings, IITC, (June 2001) pages 253–354; and “Low-k Dielectrics Characterization for Damascene Integration,” by Simon Lin, et al., IEEE Conference Proceedings, IITC, (June 2001) pages 146–148.
However, dielectric materials may be extremely porous. The porous nature of dielectric material allows copper formed in the trenches and vias, without a barrier, to diffuse into an underlying layer causing the shorting of two adjacent copper lines or line-to-line leakage. Moreover, the surfaces of dielectric materials often possess openings or gaps where pores are exposed. Therefore, interconnect structures employ a barrier layer over the surface of the dielectric to protect from copper diffusing into the dielectric material. Common materials used for this barrier layer are Tantalum, Tantalum-Nitride, and Titanium Nitride.
Yet, any discontinuity, like discontinuities 130 in FIG. 1a, in a thinly applied barrier film 120 will result in the diffusion of copper atoms or penetration of plating solution into a porous dielectric 110. This diffusion can also cause copper lines to short or leakage from line-to-line to occur. As shown in FIG. 1b, prior art for highly porous dielectrics require the deposition of a thicker barrier layer 140, typically greater than 30 nm, to physically cover the exposed pores to form a continuous barrier. Nevertheless, this thicker barrier layer takes up additional volume in a via or a trench increasing the resistance by reducing the volume available for copper.